`literal/1` for expanding literal values in fragments

Ecto has a `literal/1` https://hexdocs.pm/ecto/Ecto.Query.API.html#fragment/1-literals and we need one as well (which will just pass through to theirs)

Start with the Ecto.Query.API fragment/1 documentation linked in the issue and inspect Ash's existing fragment-related query API entry points. Done means Ash exposes literal/1 for expanding literal values in fragments and passes it through to Ecto as described.

Written by the indexing model from the issue text.